You heard it here first, folks. The Sussman's next book, Software Design for Flexibility, will be out soon. Abelson talked about it at around 25:45 in this interview from last year: https://corecursive.com/039-hal-abelson-sicp/, and it's named in the documents linked at the page for the "Advanced Symbolic Systems" course that Sussman is currently teaching: https://groups.csail.mit.edu/mac/users/gjs/6.945/
It's a graduate level class, meaning the book will likely be a step up from SICP. To whet your appetites while you wait, you can find talks Sussman has given over the past several years on Youtube, where he talks about a lot of the same principles regarding flexibility.
Now that The Sussman has four mind-bending books out, we might consider replacing satori with the Theraveda Four Stages of Nirvana:
SICP - Stream Entry
SICM - Once-Returning
Functional Differential Geometry - Non-Returning
Software Design for Flexibility - Arhantship
Is there any info on when the book will be publicly released?
According to the red tape memo for the Spring 2020 class Adventures in Advanced Symbolic Programming
Readings: The readings for this subject will be taken from a variety of sources. There will be assigned readings with each problem set. The textbook for this class is currently under development; draft sections will be distributed as needed. The text book is: Chris Hanson and Gerald Jay Sussman; Software Design for Flexibility
Abelson was saying it would probably be this fall. Of course it could be delayed, but I don't think we need to fear a Knuth-tier wait.